Fly ash is a by-product of burning coal, typically generated during the production of electricity at coal-fired power plants. Fly ashes are mainly composed by aluminosilicates partially vitrified, as well as mineral phases such as quartz, hematite, maghemite, anhydrite and so on which had been present as impurities in the original coal. ASTM C 618-85 (“Standard specification for fly ash and raw calcinated natural pozzolan for use as a mineral admixture in Portland cement concrete”) has classified fly ash into two classes, Class C and Class F, depending on the total sum of silica, alumina and ferric oxide present. Class F contains more than 70% of the above oxides and Class C contains less than 70% but more than 50%. Class F fly ash is typically low in calcium oxide (<8%) whereas Class C has a higher content being sub-classified in two categories: Class Cl (8-20% CaO) and Class CH (>20% CaO). Therefore, Class F fly ash is not usually considered as a cementitious material by itself because, due to its low calcium oxide content, it cannot be agglomerated after hydration to produce bonding strength in the final product, contrary to Class C fly ash.
Fly ash is a by-product that has to be used and consumed to reduce its environmental impact. Nowadays, it has mainly been used as a partial substitute in ordinary Portland cement due to its pozzolanic reactivity. However, there is a limitation in the replaced quantity because the pozzolanic reaction rate is very low at room temperature causing initial low strength and fast neutralization.
Recently trials have been carried out to increase the pozzolanic reaction rate by using activators such as alkaline and alkaline earth compounds (ROH, R(OH)2), salts from weak acids (R2CO3, R2S, RF) and silicic salts type R2O(n)SiO2, where R is an alkaline ion from Na, K or Li. However, either the activation efficiency is not enough or there are some undesired interactions between ordinary Portland cement and activators, which causes rheological and/or mechanical problems. This fact promotes the use of additional components, mainly admixtures, which increases the complexity of the formulation and makes worse the technological development of these products.
The high amount of lime CaO in fly ash type C provides the waste product with intrinsic cementitious properties. On the other hand, fly ash type F does not by itself develop any strength on hydration, and an activation of the product is requested to ensure that strength development will take place on hydration. A major advantage to prefer fly ash type F rather than fly ash type C is the high availability in large quantities of fly ash type F and its lower market price. Since transportation costs of industrial wastes would be a key issue for the cost effectiveness of the final product or binder, the selection of fly ash type F is guided by its availability in large quantities and its dense geographic distribution.
For many years, many formulations and processes have been proposed to activate fly ash or industrial wastes in order to use it as a cementitious material. U.S. Pat. Nos. 5,435,843 and 5,565,028 described the activation of Class C fly ash at room temperature with strong alkali (pH>14.69) to yield cementitious properties. Even though there is no express mention of Class F fly ash use in these patents, the cement containing Class C fly ash according to these patents has limited application due to the corrosive properties (pH>14.6) of the used activators.
EPO Patent No. 0858978 discloses that high volumes of activated Class C Fly ash (>90%) may be used as a cementitious binder. The binder contains a mix of Class C and Class F Fly ashes wherein the dosage of Class F fly ash has to be limited up to 60% due to its low reactivity. In this case, Class F Fly ash is mentioned but it is used together with clinker and admixtures like citric acid, borax, Boric acid, which are very expensive, and KOH, which is corrosive (pH>13). Furthermore, formulations get complex because the high number (>6) of presented components.
In a similar way, U.S. Pat. No. 5,482,549 describes a cement mixture containing at least 2% by weight of Portland cement clinker, 2-12% by weight of sodium silicate, fly ash and blast furnace slag. The patent specifies that the fly ash has to be ground to a specific surface of more than 500 square meters per kg which is very important and yields high manufacturing costs (energy consumption, handling, etc.). Furthermore, this document doesn't mention the use of Class F fly ash.
Xu et al., “The activation of Class C-, Class F-Fly Ash and Blast Furnace Slag Using Geopolymerisation”, 8th CANMET/ACI International Conference on Fly Ash, Silica Fume, Slag and natural Pozzolans in Concrete, Las Vegas, Calif., USA (2004), shows that Class F fly ash can only be properly activated when using a highly alkaline soluble silicate solution. Following this line, U.S. Pat. No. 5,601,643 proposes an invention related with chemically-activated fly ash cementitious materials, preferably Class F Fly ash, where high content of alkali metal and/or alkaline earth metal silicate are used to obtain high strength cementitious mixtures. However, this invention has a limited application because: 1) a high curing temperature is need, 2) a high pH (>14, corrosive products) is required and therefore, safety conditions are necessary to handle the mixture and 3) the cost of the mixture is high due to the high quantities of soluble silicates and alkalis used. Furthermore, formulations related with high alkalis content and high pH cause alkali-leaching problems and efflorescence due to the overdose of activators. The overdose of activators is due to Class F fly ash that is considered as a binder and not as active filler, which requires less alkaline dosage for being activated.
Skvara et al. (Ceramics Slicaty 43-1999) described alkali activated mixtures of slag and fly ash using sodium silicates and sodium hydroxides at high dosages (SiO2/Na2O located from 0.6 to 1.6) on pastes. PCT patent publication no. WO 2005/09770 discloses alkali activated mixtures of slag and fly ash in the form of pastes using gypsum additions in the anhydrite form and superplasticizers to achieve significant strength development. Most if not all of these studies present results based on pastes (sometimes on standard mortars) with limited or no industrial interest and most of the time very high costs.
The activation of the various latent pozzolanic materials (e.g. slag, clay, fly ashes, flues, natural pozzolan) is described using various sources of alkalis salts (silicates, carbonates, hydroxides) but most of the time, the respective amounts of the alkali sources is not detailed. However, experience has shown, that the source of the alkali for activation plays an important role and that any combination does not provide the same results. Finally, optimizing the quantity and the source of the chemical activator is highly relevant in order to control the cost of the final concrete product. Furthermore, most of the publications emphasize that curing of these pastes should occur at elevated temperatures (above 40° C.) or need a preliminary heat treatment for some hours at temperatures located between 60° C. and 100° C. Considering real industrial construction material poses other problems than trying to activate latent hydraulic material to develop some strength in pastes.
For concrete applications, using conventional amounts of sand and aggregates, the problem is different since workability and strength development mechanisms are clearly affected by the aggregates and the industrial mixing conditions. Therefore, the invention intends to describe new industrial construction materials, mainly concrete mix designs that can be used in many structural applications (ready mix, pre-cast). The invention consists in providing an alternative to conventional Portland cement based concrete. Furthermore, the content and the nature of the chemical activators have to be optimized in order to enable effective strength development, cost effective final concrete and to avoid leaching and lixiviation problems related to unreacted excess chemical activators.
The aim of the invention is to remedy to the above drawback by providing industrially applicable concrete compositions with the following advantages:                environmental friendly;        easy to formulate involving limited number of components;        safe and easy to handle and to prepare with conventional equipment;        cost effective;        controlled workability without additions of organic superplasticizers;        ability to be prepared on the construction site; and        no specific curing conditions.Typically, the invention doesn't aim to use any cement or cement related compounds (like cement kiln dusts for instance). The advantage not to use cement in the formulation of the binder is mainly based on the objective of simplicity and polyvalence of the invention. Cement or the like additions in the formulation will lead to additional problems of interactions with the chemical activators that need to circumvent by further specific chemicals etc, special curing conditions, etc. The objective of early strength development, as well as the universal property of the binder will be very difficult to achieve. Finally, the ecological advantages of the product according to the invention will be reduced since cement, clinker or cement kiln dust additions are correlated to additional CO2 emissions. It will be seen in the following description that none of the prior art present the technical features and none of the prior art have all advantages provided by the present invention.        